Heterogeneous chlorine activation on stratospheric aerosols and clouds in the Arctic polar vortex

Abstract

Abstract. Chlorine activation in the Arctic is investigated by examining different parameterizations for uptake coefficients on stratospheric aerosols, high-resolution in-situ measurements and vortex-wide satellite observations. The parameterizations for heterogeneous chemistry on liquid aerosols are most sensitive to temperature with the reaction rates doubling for every 1 K increase in temperature. However, differences between the currently available parameterizations are negligible. For Nitric Acid Trihydrate particles (NAT) the major factors of uncertainty are the number density of nucleated particles and different parameterizations for heterogeneous chemistry. These two factors induce an uncertainty that covers several orders of magnitude on the reaction rate. Nonetheless, since predicted reaction rates on liquid aerosols always exceed those on NAT, the overall uncertainty for chlorine activation is small. In-situ observations of ClO x from Arctic winters in 2005 and 2010 are used to evaluate the heterogeneous chemistry parameterizations. The conditions for these measurements proved to be very different between those two winters with HCl being the limiting reacting partner for the 2005 measurements and ClONO 2 for the 2010 measurements. Modeled levels of chlorine activation are in very good agreement with the in-situ observations and the surface area provided by Polar Stratospheric Clouds (PSCs) has only a limited impact on modeled chlorine activation. This indicates that the parameterizations give a good representation of the processes in the atmosphere. Back-trajectories started on the location of the observations in 2005 indicate temperatures on the threshold for PSC formation, hence the surface area is mainly provided by the background aerosol. Still, the model shows additional chlorine activation during this time-frame, providing cautionary evidence for chlorine activation even in the absence of PSCs. Vortex-averaged satellite observations by the MLS instrument also show no definite connection between chlorine activation and PSC formation. The inter -and intra-annual variability of vortex-average HCl and HNO 3 based on MLS observations is examined for the Arctic winters 2004/2005 to 2010/2011. These observations show that removal of HCl and HNO 3 from the gas-phase are not correlated. HNO 3 loss exhibits great inter-annual variability depending on prevailing temperatures while HCl loss is continuous through December without considerable inter-or intra-annual variability. Only the recovery of HCl in late winter depends on the level of denitrification. Hence, the occurrence of HNO 3 containing PSC particles does not seem to have a significant effect on the speed of initial chlorine activation on a vortex-wide scale.

Contribution of liquid, NAT and ice particles to chlorine activation and ozone depletion in Antarctic winter and spring

“…An alternative parameterisation for reactions on liquid particles was reported by Shi et al (2001), which results however in a heterogeneous reactivity very close to the one employed here (e.g. Wegner et al, 2012).…”

“…They concluded that the surface area provided by PSCs did not significantly impact chlorine activation rates in these both winters. Wohltmann et al (2013) showed, based on simulations of the Arctic winter 2009/2010 with the Lagrangian model ATLAS, that liquid aerosols alone allow to explain the observed mixing ratios of active chlorine and ozone from Microwave Limb Sounder (MLS), Atmospheric Chemistry Experiment -Fourier Transform Spectrometer (ACE-FTS) and in situ measurements.…”

“…the reformation of HCl and ClONO 2 , increases the rate at which heterogeneous reactions must proceed to keep pace with deactivation. Wegner et al (2012) examined the importance of the cold binary aerosols for chlorine activation in the Arctic winters 2004/2005 and 2009/2010 with the help of in situ observations. They concluded that the surface area provided by PSCs did not significantly impact chlorine activation rates in these both winters.…”

Comparisons of polar processing diagnostics from 34 years of the ERA-Interim and MERRA reanalyses

“…Although some recent studies have suggested that liquid PSCs play a dominant role in activating chlorine (e.g., Wegner et al, 2012;Wohltmann et al, 2013), the formation temperatures of solid nitric acid trihydrate (NAT, Hanson and Mauersberger, 1988) and ice particles remain convenient thresholds for the initiation of chlorine activation processes. In this study, we examine daily 12:00 UT minimum temperatures and calculations of area with temperatures below PSC thresholds (henceforth T min and A PSC , respectively).…”

Fundamental differences between Arctic and Antarctic ozone depletion

“…The relative roles and temperature dependencies of surface reactions on and in water ice, nitric acid trihydrate ice, and supercooled liquid sulfuric acid−water−nitric acid and binary sulfuric acid−water particles at background levels have been research topics since the late 1980s, and some important questions have recently reemerged. Some recent studies have suggested that heterogeneous chemistry taking place on background particles of sulfuric acid and water is sufficient to explain nearly all of the chlorine activation (17)(18)(19) and ozone losses in both the Arctic and Antarctic (17) without any need for temperatures below −78°C or formation of polar stratospheric clouds, which would represent a major change in understanding. The potential for liquid binary aerosols to play some role in ozone loss under cold conditions was first identified decades ago (20,21).…”